This invention relates to apparatus and methods for linearizing the response of a high power amplifier (HPA).
In both communications and radar systems, the performance of the systems is limited by the linearity (or lack thereof) of the high power amplifiers used to transmit the RF output signals. Nonlinearities in the response of the high power amplifiers create distortions that limit the dynamic range of the amplifiers. Efforts to correct this problem, decrease the amplifier efficiency while increasing the hardware complexity and cost.
In known power amplifier systems, while it is the rf waveform that gets distorted, the conventional corrective measures (such as a compensating predistortion equalizer) are applied on, and to, the baseband signals due to speed limitations of known semiconductor electronics. Such baseband schemes are fundamentally constrained to partial correction to weak nonlinearity.
A known scheme may be explained with reference to FIG. 1 which shows a prior art digital baseband predistortion correcting arrangement. Digital baseband data (Iin and Qin) are supplied to a digital pre-distorter circuit 13 whose output O13 is supplied to a digital-to-analog converter (DAC) 15 to produce an (unmodulated) output O15 which is supplied to an analog up-converter circuit 17 in which the base band signal (at O15) is modulated (by means of an analog local oscillator 18) so as to produce an output O17 which is an RF modulated signal. The RF modulated output O17 is then fed to an analog bandpass filter 19 whose output O19 is then fed to a high power amplifier (HPA) 21. A portion O21 of the HPA 21 output (which is an RF modulated signal) is fed back to an analog bandpass filter 23 whose output O23 (which is still an RF modulated signal) is supplied to an analog down-converter 25 to produce a demodulated output O25 which is fed to an analog-to-digital converter (ADC) 27 whose digitized output O27 is fed to baseband predistorter circuit 13 for determination and introduction of any needed modification and/or correction to the signal (O13) being fed via Dac 14, up-converter 17 and BPF 19 to the HPA 21. Thus, although a significant portion of the non-linearities occur in the RF section of the system, the corrections and modifications are conducted at the baseband level. This is problematic because the corrections are not directly introduced on the RF signal and there is a significant time delay between the signal sensed at the output of the HPA and the application of a corresponding signal to the baseband predistorter and the subsequent generation of a “pre-distorter” signal to be supplied to the HPA.